Resonance Fluorescence of an InGaAs Quantum Dot in a Planar Cavity Using Orthogonal Excitation and Detection

Summary

Resonant excitation of a single self-assembled quantum dot can be achieved using an excitation mode orthogonal to the fluorescence collection mode. We demonstrate a method using the waveguide and Fabry-Perot modes of a planar microcavity surrounding the quantum dots. The method allows complete freedom in the detection polarization.

Abstract

The ability to perform simultaneous resonant excitation and fluorescence detection is important for quantum optical measurements of quantum dots (QDs). Resonant excitation without fluorescence detection – for example, a differential transmission measurement – can determine some properties of the emitting system, but does not allow applications or measurements based on the emitted photons. For example, the measurement of photon correlations, observation of the Mollow triplet, and realization of single photon sources all require collection of the fluorescence. Incoherent excitation with fluorescence detection – for example, above band-gap excitation – can be used to create single photon sources, but the disturbance of the environment due to the excitation reduces the indistinguishability of the photons. Single photon sources based on QDs will have to be resonantly excited to have high photon indistinguishability, and simultaneous collection of the photons will be necessary to make use of them. We demonstrate a method to resonantly excite a single QD embedded in a planar cavity by coupling the excitation beam into this cavity from the cleaved face of the sample while collecting the fluorescence along the sample's surface normal direction. By carefully matching the excitation beam to the waveguide mode of the cavity, the excitation light can couple into the cavity and interact with the QD. The scattered photons can couple to the Fabry-Perot mode of the cavity and escape in the surface normal direction. This method allows complete freedom in the detection polarization, but the excitation polarization is restricted by the propagation direction of the excitation beam. The fluorescence from the wetting layer provides a guide to align the collection path with respect to the excitation beam. The orthogonality of the excitation and detection modes enables resonant excitation of a single QD with negligible laser scattering background.

Introduction

Resonant excitation of a single quantum emitter combined with fluorescence detection was a long-term experimental challenge mainly due to the inability to spectrally discriminate the weak fluorescence from the strong excitation scattering. This difficulty, however, has been successfully overcome in the past decade by two different approaches: dark-field confocal excitation based on polarization discrimination^1,2,3,4,5, and orthogonal excitation-detection based on spatial mode discrimination

Protocol

Caution: Please be aware of the possible dangers of laser scattering during the alignment. Wear proper safety goggles for protection. To facilitate the alignment process, an infrared viewer (IR-viewer) is necessary. An IR-sensitive fluorescent card is also helpful but not necessary.

1. Sample Preparation

1. Use a diamond scribe to make a miniscule scratch on the edge of the top surface of the sample at the desired location of the cleave. Use two pairs of flat-ended tweezers to hold the sample on both sides of the scratch. Apply an outward-rotating torque with the tweezers and the sample will cleave.
   NOTE: A long scratch is u....

Results

Figure 1 shows one particular realization of the necessary equipment to accomplish resonant excitation of a single quantum dot. Other realizations are possible, but the critical components are: an excitation path to couple to the waveguide; a collection path to guide fluorescence to detectors; a confocal excitation path to excite along the collection path; and an illumination path to enable imaging of the sample surface.

Discussion

The critical steps in the protocol are: the mode-matching and alignment of the excitation beam to the waveguide mode; and proper alignment and focusing of the collection optics. The most difficult parts of these steps are the initial alignment; optimizing the coupling of an already aligned setup is relatively straightforward. Overlapping the collection and excitation areas is a step that is simple with the ability to image the sample on the camera, but is very difficult without this capability. In order to have high-qual.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Tunable external cavity diode laser	Toptica Photonics	DL-Pro
Closed-cycle cryostat	Montana Instruments	Cryostation
Spectrometer, 750 mm focal length	Princeton Instruments	SpectraPro 2750
Thermoelectrically cooled charge-coupled device	Princeton Instruments	Pixis 100BR-eXcelon
HeNe laser	JDSU	1125P
Infrared sensitive camera	Sony	NEX-5TL	IR blocking filter removed
Power meter and detector	Newport	1918-C, 918D-IR-OD3
Adjustable aspheric fiber collimator	Thorlabs	CFC-8X-A
Air-Spaced Doublet Collimator	Thorlabs	F810APC-842
Protected Silver Mirrors x 5	Thorlabs	PF10-03-P01
Flip mounts x 2	Thorlabs	FM90
Aspheric condenser lens, f = 20 mm; K1	Thorlabs	ACL2520-B
Best form spherical lens, f = 50 mm; E2, L1, L2, K2	Thorlabs	LBF254-050-B
Best form spherical lens, f = 100 mm; E1, L4, K3, K4	Thorlabs	LBF254-100-B
Best form spherical lens, f = 200 mm; Lspec, Lcam	Thorlabs	LBF254-200-B
Plano-convex lens, f = 400 mm; L3	Thorlabs	LA1172-B
Molded glass aspheric lens, f = 8 mm; Eobj	Thorlabs	C240TME-B
Precision asphere, f = 10 mm; Lobj	Thorlabs	AL1210-B
Longpass Filters, 800 nm, x2	Thorlabs	FEL0800
Non-polarizing beam splitter cube (NPBS)	Thorlabs	BS029
Pellicle beam splitter	Thorlabs	BP108
Polarizer	Thorlabs	LPNIRE100-B
Light emitting diode, 940 nm	Thorlabs	M940D2